Acoustic well logging is a well developed art, and details of acoustic logging tools and techniques are set forth in A. Kurkjian, et al., “Slowness Estimation from Sonic Logging Waveforms”, Geoexploration, Vol. 277, pp. 215-256 (1991); C. F. Morris et al., “A New Sonic Array Tool for Full Waveform Logging,” SPE-13285, Society of Petroleum Engineers (1984); A. R. Harrison et al., “Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole and Dipole Source . . . ” SPE 20557, pp. 267-282 (September 1990); and C. V. Kimball et al., “Semblance Processing of Borehole Acoustic Array Data”, Geophysics, Vol. 49, pp. 274-281 (March 1984), all of which are hereby incorporated by reference herein.
An acoustic logging tool may include an acoustic source (transmitter), and a set of receivers that are spaced apart from each other. An acoustic signal may be transmitted by the acoustic source and received at the receivers. Measurements can be repeated periodically as the tool is drawn up (or down) the borehole. The acoustic signal from source may travel through the formation adjacent the borehole to the receiver array, and the arrival times and perhaps other characteristics of the receiver responses are recorded. Received signals may include compressional wave (P-wave), shear wave (S-wave), and Stoneley waves. Received data may be processed to find formation characteristics, including slowness (the inverse of acoustic speed), from which pore pressure, porosity, and other formation property determinations can be made.
Typically, the processing involves digitizing the received signal at a desired sampling rate and then processing the digitized samples according to desired techniques. Examples may be found in the references cited above, as well as in articles such as A. R. Harrison et al., “Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole and Dipole Source . . . ” SPE 20557, pp. 267-282 (September 1990). In some tools, the acoustic signals may even be used to image the formation.
Traditional compensated bonding analysis may identify casing arrivals using the signals from two transmitters and two receivers, depending on the peak amplitude of first arrival signals. However, sensitivity to signal amplitudes may induce failure when the signals are weak. Detection of casing arrival signals may also require additional input from an operator, including gating setup and gain adjustments.